This invention relates to magnetic resonance(MR) imaging apparatus and, more particularly, to an MR imaging apparatus comprising an RF shield for placement between an RF coil and a set of gradient coils.
An MR imaging apparatus comprises a magnet means for applying a static magnetic field to a human body, a gradient coil means for providing a magnetic gradient which is superimposed on the static magnetic field and an RF coil means for transmitting an RF pulse(typically about 10 MHz) to the body and receiving an echo data from the body.
The gradient coil means typically utilizes a set of three gradient coils Gx, Gy and Gz to obtain spatially-selective information. Each of the, these gradient coils generally contains a multiplicity of turns of conductive wire.
RF fields lose a significant portion of their energy if these fields impinge upon the conductive wires of the gradient coils. Any RF power loss, in the gradient coils or otherwise, appears as a lowering of the quality factor Q of the RF coil and consequently appears as a lowering of the signal to noise ratio(SNR) attainable in the imaging device.
Accordingly, it is highly desirable to prevent penetration of the RF field into the gradient coils.
A shield is typically placed between the RF coil and the gradient coils.
The RF shield must, however, be substantially transparent to the gradient magnetic fields and therefore must prevent inducement of any significant shield currents at gradient frequencies (typically less than about 10 KHz) to prevent temporally-dependent and/or spatially dependent magnetic field inhomogeneities which causes an adverse affect on the resulting image.
U.S. Pat. No. 4,879,515, issued Nov. 7, 1989 to Roemer et al. discloses a radio-frequency(RF) shield, for use interposed between a set of gradient coils and an RF coil, for example, "birdcage" coil in an NMR imaging device, which is formed of a conductor-dielectric-conductor laminate.
Each conductive layer of the laminate has a plurality of conductive streamline loop portions which are each defined by nonconductive cut lines parallel to RF current flow induced therein and a radial cut line separating each loop portion into a generally "C"-shaped conductive loop portion.
According to Roemer's Patent, the high frequency RF current can flow along the C-shaped conductive loop portion through capacitors made along the radial cut line and the low frequency gradient current flow can be prevented because of the radial cut line.
However, first of all, an impedance of a circuit, along which the RF shield current flows, can not be reduced so much because of the location of the capacitor, thus the impinging of the RF field upon the gradient coils can not be prevented perfectly. Secondly, because of the laminate, a construction of the RF shield is complicated to increase a cost for making the RF shield. Thirdly, other type of an RF coil can not be used because the nonconductive cut line are defined for the predetermined RF coil, for example, "birdcage" type of RF coil.